In the course of erecting steel structures, a thick coating of inorganic material is commonly applied to the metallic structural elements to achieve a number of objectives including fire retardance, improved appearance and sound deadening. While several types of formulations have been used for these purposes over the years by means of a variety of techniques, one successful system consists of spraying onto the steel surfaces settable aqueous mixes as disclosed in U.S. Pat. No. 4,751,024, assigned to the assignee of the present invention. That patent teaches sprayable cementitious fireproofing compositions containing shredded expanded polystyrene as a lightweight aggregate.
In order to be suitable for such use, coating mixes, both in the wet and dry state, must possess a number of important properties. They must be able to hold a quantity of water that renders them capable of being pumped easily and to great heights, yet they must retain a consistency sufficient to prevent segregation or settling of ingredients and permit adequate "yield" or coverage of the steel surface at a given thickness. The coating mixes, furthermore, must obviously adhere to steel surfaces, both in the slurried state and in the dry state. Also, the mix must set without undue expansion or shrinkage which could result in the formation of cracks that would seriously deter from the insulative value of the dry coating.
Published European Patent Application No. 90308027.3, the disclosure of which is incorporated herein by reference, teaches that sprayable cementitious fireproofing compositions containing shredded polystyrene having a particular particle size distribution result in compositions having better uniform consistency and quality in terms of pumpability, hangability and yield. In the aforementioned European Patent Application it is disclosed that in shredding the polystyrene to achieve the desired particle size distribution, important considerations include: the degree of fusion in the expanded polystyrene board used as a starting material for the aggregate, the shredding rotor speed, the roughness of the brushes on the shredder and the tolerance between the brushes. With particular reference to the shredder brush parameters, it has been found that reproducible particle size distribution becomes difficult as the brushes wear; in particular, the proportion of particles we define as "fines", i.e., less than about 325 mesh, increases to an unacceptable level.
Furthermore, the machine disclosed in the aforementioned European Patent Application is difficult to maintain and operate; the wire brushes are difficult to clean, and the whole apparatus has been found prone to overheating. When this occurs the expanded polystyrene in the shredder melts and gums up the wire brushes, necessitating their replacement. This is a bothersome and costly proposition.
It therefore would be desirable to produce a shredding machine that, over long periods of operation, will produce a shredded aggregate having a consistent particle size distribution; that is more durable; that is easy to maintain; and that can be made easily and inexpensively.